In a system for transmitting/receiving radiofrequency signals, the receiving stage comprises an antenna allowing for the reception of a signal that will be transmitted to an amplifier circuit. Elements such as filters may be inserted between the antenna and the amplifier circuit. The main purpose of the amplifier is to bring the signal to a level suitable for the demodulation circuit.
In the case of receivers for wireless telecommunications infrastructure, there is a great need to amplify the received signal with very low noise and high linearity.
Low-noise amplifiers (LNAs) simultaneously demand a relatively high gain depending on the standard used, low noise, good input and output matching and solid stability with regard to the operating current.
Usually, low-noise amplifiers are equipped with an amplifier circuit generally comprising active elements such as transistors, with an inductive degeneration element, with an inductive output element and, additionally, an inductive element for matching the input impedance (or inductive input element). The inductive input element allows, to the first order, the imaginary part of the input impedance of the amplifier to be cancelled out, in order to have an input impedance corresponding to the impedance of the system connected to the input (generally 50 ohms for an antenna) at the operating frequency of the amplifier. This allows an optimal transfer of energy.
In order to minimize the noise at frequencies of the order of a GHz, the inductive input element advantageously has a quality factor “Q” that is the highest possible. An increase in the surface area of the inductive element, with respect to a more compact inductive element of the same inductance, makes it possible to increase the quality factor “Q”. It is for this reason that usually, for demanding noise specifications, the inductive input element is not integrated on the same, for example silicon, substrate as the amplifier circuit, owing to the large potential size thereof.
The inductive input element is consequently usually positioned on the outside of the integrated amplifier circuit, for example on a printed circuit board. These external inductive elements, also called discrete elements, have a high quality factor “Q”.
Currently, in particular within the framework of the Long-Term Evolution (LTE) telecommunications standards, the specifications for a low-noise amplifier in a frequency band of 2.62 GHz to 2.69 GHz are, for example, a gain of about 13 dB, a noise factor (NF) of about 1 dB, a third-order input interception point (IIP3) of about 5 dBm and an isolation S12 of about −20 dB.
The isolation parameter S12 represents the way in which a variation applied to the output of a system is felt at the input thereof, and vice versa. It is then possible to speak of the robustness of the amplification system. The IIP3 parameter is representative of the linearity of the amplification.